Electromechanical watthour meters typically include an aluminum disk supported on a shaft and driven, or rotated, by an electromagnetic field at a speed proportional to the electric power being consumed by a load. Geared dials, or cyclometer discs, integrate the disk motion to indicate the total energy consumed, typically in kilowatt hours. In addition to a kilowatt hour measure of consumption, some meters include demand registers, time-of-use registers, or other registers for recording other energy consumption related data. Such registers can be either mechanical or electrical. Electromechanical watthour meters and registers are well known and commercially available from General Electric Company, Somersworth, N.H.
The aluminum disk is connected to a shaft which is supported on bearings. A voltage coil and a current coil are positioned on respective, opposite sides of the disk. In a residential metering application, two power lines extending from a distribution transformer are utilized to deliver energy from the power system to the residence. The meter voltage coil is electrically connected across the lines, and the current coil is electrically connected in series with one of the lines. A permanent magnet, sometimes referred to as a retarding magnet, is spaced from the voltage and current coils and is positioned so that its poles are on opposite sides of the disk.
In operation, the magnetic fields from the voltage and current coils induce currents in the rotational disk, and the currents in the disk set up associated magnetic fields. The magnetic fields of the voltage and current coils couple with the magnetic fields of the induced disk currents, resulting a rotational torque being applied to the disk. In addition, the fields produced by the permanent magnet generate a retarding torque on the disk.
Using the above described configuration, a highly accurate measurement of energy consumption is generated. Specifically, each complete revolution of the disk corresponds to a predetermined quantity of energy consumed. The meter register, by precisely measuring disk revolutions, generates an accurate measure of energy consumption.
While it is important to generate an accurate measurement of energy consumption, it also is desirable to employ a meter structure that allows energy consumption to be measured in a cost efficient manner. In attempting to achieve a low cost meter, however, accuracy of the meter must not be adversely affected, nor should operational energy requirements of the meter itself be increased, e.g., meter energy consumption should not be increased in order to lower meter cost. Substituting lower cost meter components for components that are critical in generating accurate energy consumption measurements will typically result in less accurate metering. For example, it is important that the retarding magnet generate a constant retarding torque, or drag, on the disk under various conditions, including a variety of environments having a wide range of temperatures. An electricity meter for use in Florida should preferably be manufactured in an identical manner, and use the same components, as a meter for use in Alaska. Therefore, the retarding magnet must have a low temperature coefficient in order to be relatively insensitive to temperature changes.
One alloy used for the retarding magnet, known as Alnico-5, has a low temperature coefficient, but is expensive. Less expensive magnetic material, e.g., ferrite, typically has a higher temperature coefficient than Alnico-5, and due to the importance of maintaining a constant drag on the disk, Alnico-5 is typically selected over ferrite for the retarding magnet. The Alnico-5 magnet is usually secured to the meter frame by an expensive nickel-iron keeper, so that a substantially constant drag is applied to the disk over a wide range of ambient temperatures. This allows the desired accurate metering to be achieved.
It would be desirable to employ a retarding magnet that enables more cost efficient metering of energy consumption. Such retarding magnet assembly, however, should not adversely affect the accuracy of energy consumption metering nor increase the operational energy requirements of the meter itself.